Cold Interstellar Matter : Modelling- and Experimental- based Studies

CIMMES is a project funded by the ANR (Programme blanc) for 3.5 years from 15/01/2012 to 14/07/2015.

The CIMMES project will study the evolution of cold cosmic dust within the framework of the current era of dust exploration in the far infrared and submillimeter (FIR/submm) wavelength region. Observations in the far infrared and submillimeter range trace cold dust and allows us to detect pre-stellar clouds prior to their collapse, and thus to study the first steps of star formation, and to estimate the dust mass in cold interstellar clouds. A good knowledge of the dust physical, chemical and optical properties, and its thermal emission is therefore needed in order to correctly interpret observational data.
The emission from cold dust in the Interstellar Medium (ISM) is usually expressed as the product of the dust mass Md, the mass absorption coefficient κλ0 at some reference
wavelength, the Planck function Bλ(T) and the spectral index β, (λ/λ0)-β. Simple semi- classical models of absorption provide a temperature independent asymptotic value β = 2,
which is commonly used in astrophysical studies. Before the launch of the Herschel and Planck satellites the, relatively few, observations at FIR and submm wavelengths revealed that the dust emission was not easy to interpret. Principal among the observed anomalous behaviours are the far-infrared excess observed by the instrument FIRAS on board the COBE (Cosmic Background Explorer) satellite and the β-T anti-correlation of the spectral index, β, and the dust temperature, T, derived form the observations of the two balloon-born missions PRONAOS and ARCHEOPS. The first results from the Herschel and the Planck missions are confirming these dust emissivity variations and indicates that they are ubiquitous (see the A&A Special Issue vol. 518 and the Planck Early Results).Recent results from Herschel and Planck illustrate the richness of the information available in these observations for the study of dust and the ISM of our Galaxy and external galaxies. However, they also underline the striking lack of laboratory data on cosmic dust analogues that are needed to interpret the observations and to model the FIR dust emission. For the last 15 years, only a few laboratory studies were dedicated to the FIR/submm spectral range and to temperature effect. Consequently, the dust optical properties at these long wavelengths are often extrapolated from those measured at shorter wavelengths and this is almost certainly not valid.

In this context, our collaboration has developed joint observational/theoretical and experimental approach aiming at studying cold dust evolution in the ISM. The CIMMES project gathers together astrophysical institutes (the IRAP and the IAS) having expertise in observational data interpretation and laboratory astrophysics (through the
ESPOIRS experimental setup, Etudes Spectroscopiques des Propriétés Optiques InfraRouge et Submillimétrique d’analogues de grains interstellaires) and chemistry and physics institutes (the LPCNO and the UMET) having expertise in nanograins synthesis and characterization and extraterrestrial matter.

Within the interdisciplinary collaboration between the astrophysicists, chemists and mineralogists that we have built this project around, we will synthesise dust analogues (silicates, carbonaceous dust and ices) and characterize their optical properties in the FIR/submm at low temperature. The physico-chemical properties of cosmic dust and the processes/local conditions that govern these properties will be constrained by the interpretation of the observations from Herschel and Planck dedicated programs in which the IRAP and the IAS are involved. The data interpretation will use the models developed within our team. This includes a new physical model of the dust FIR/submm emission and models of the dust properties and evolution.

The CIMMES project will study the evolution of cold cosmic dust within the framework of the current era of dust exploration in the far infrared and submillimeter (FIR/submm) wavelength region.

The Herschel and Planck space missions are now furnishing abundant data in the far- infrared (FIR) to millimetre (mm) wavelength (FIR-mm) range that is yet awaiting a detailed interpretation. The astronomical community uses FIR-mm dust emission observations to trace dense matter within the Galaxy, from diffuse clouds to pre-stellar cores, and thus to study the first steps of star formation, and estimate the dust mass in cold interstellar clouds, but also to trace cold matter within distant galaxies. For the astronomical community, understanding the nature of the FIR-mm dust emission is proving absolutely critical for the subtraction of the foreground `contamination’ by dust and gas in the interpretation of the extragalactic and cosmological data. However, our current knowledge of the nature of the large, cold interstellar grains is incomplete and this area still remains a major challenge to the interpretation of the new FIR-mm observations of the dust thermal emission at long wavelengths.

The key questions driving our project are:

• What are the optical properties of cosmic dust at low temperature in the FIR/submm?

• What is the physics of the dust FIR/submm emission?

• What are the physico-chemical properties of cosmic dust and what processes/local conditions govern them?

This project is based on a joint observational/theoretical and experimental approach aimed at studying the cold dust evolution in the ISM.

Two axes shape the project structure:

the first, based on astronomical observations and modelling, will be accomplished at the IAS and at the IRAP

the second, based on experiments and theoretical work, will be performed at the IRAP, at the LPCNO and the UMET.

We have developed modelling tools (
DustEM, DustEV, DustPROP, the TLS model) in parallel with an experimental setup (
ESPOIRS based at the IRAP, Etudes Spectroscopiques des Propriétés Optiques InfraRouge et Submillimétrique d’analogues de grains interstellaires) and fruitful collaborations with national and international chemists and physicists.